What is the initial approach to managing a viral infection in a healthy adult?

How to Identify a Viral Infection in a Healthy Adult

The diagnosis of viral infection relies primarily on clinical presentation combined with targeted laboratory testing when specific viral identification is needed for treatment decisions or infection control, rather than routine testing for all suspected viral illnesses. 1

Clinical Differentiation: Viral vs. Bacterial Infection

Key Clinical Features Suggesting Viral Etiology

For respiratory infections, look for gradual onset with upper respiratory symptoms (rhinorrhea, sore throat, nasal congestion) without high fever or severe systemic symptoms. 1

• Symptoms lasting less than 10 days without worsening suggest viral rather than bacterial infection 1
• Absence of "double-worsening" pattern (where symptoms improve then worsen within 10 days) favors viral etiology 1
• Lack of severe symptoms such as high fever (>38°C) with purulent discharge for 3+ consecutive days supports viral diagnosis 1

Laboratory Markers for Differentiation

C-reactive protein (CRP) is the most useful biomarker: CRP <20 mg/L makes bacterial pneumonia unlikely, while CRP >100 mg/L suggests bacterial infection. 1

• CSF lactate <2 mmol/L effectively rules out bacterial meningitis and supports viral etiology 2, 3
• For suspected meningitis/encephalitis, characteristic CSF findings include lymphocytic pleocytosis (5-1000 cells/μL), mildly elevated protein (<0.6 g/L), and normal or slightly low glucose with CSF/plasma glucose ratio >0.36 2

When to Perform Specific Viral Testing

Respiratory Infections

Rapid antigen testing for influenza should be performed when results would change management (i.e., consideration of antiviral therapy within 48 hours of symptom onset), though sensitivity is limited at 50-70% in adults. 4

• For hospitalized patients with suspected pneumonia, obtain lower respiratory tract samples (endotracheal aspirates preferred over bronchoalveolar lavage) rather than nasopharyngeal swabs 4
• Multiplex PCR panels for respiratory viruses are recommended for critically ill patients with suspected viral pneumonia 4, 1
• Rapid diagnostic testing should be performed promptly on hospitalized patients with suspected RSV, parainfluenza, or adenovirus to facilitate appropriate infection control precautions 4

CNS Infections

All patients with suspected viral encephalitis must have CSF PCR testing for HSV-1, HSV-2, VZV, and enteroviruses, as these tests identify 90% of cases due to known viral pathogens. 4, 2, 3

• Throat and rectal swabs for enterovirus should be obtained in all suspected viral encephalitis cases 4
• If initial CSF is normal but clinical suspicion remains high, repeat lumbar puncture in 24-48 hours, as approximately 5-10% of viral encephalitis cases have initially normal CSF 2, 3
• HIV testing should be performed on all patients with encephalitis regardless of perceived risk factors 4

Practical Diagnostic Approach

Step 1: Clinical Assessment

• Document symptom duration, pattern of progression, and severity
• Check vital signs: fever >38°C, heart rate >100, respiratory rate >24 suggest bacterial infection 1
• Identify upper respiratory symptoms (rhinorrhea, sore throat) that favor viral etiology 1

Step 2: Risk Stratification

• Elderly patients, those with COPD, diabetes, or heart failure are at higher risk for complications and bacterial superinfection 1
• The most common bacterial superinfection following viral respiratory infection is Streptococcus pneumoniae 4, 1

Step 3: Selective Testing

• For influenza-like illness within 48 hours of onset: consider rapid influenza testing only if positive result would lead to antiviral treatment 4
• For suspected bacterial pneumonia: obtain CRP and chest X-ray 1
• For CNS symptoms: perform lumbar puncture with CSF analysis and viral PCR panel 4, 2, 3
• For hospitalized patients: obtain appropriate respiratory samples before initiating empiric antibiotics 4, 1

Critical Pitfalls to Avoid

Do not rely on rapid antigen tests alone to exclude viral infection, as negative results do not rule out disease due to limited sensitivity. 4

• Avoid misdiagnosing viral infections as bacterial, which leads to unnecessary antibiotic use and contributes to antimicrobial resistance 1
• Do not delay repeat lumbar puncture in suspected viral encephalitis with initially normal CSF 2, 3
• Avoid nasopharyngeal swabs alone for COVID-19 diagnosis in hospitalized patients, as they have low negative predictive value (only 47% sensitivity in one study) 4
• Do not perform routine microbiological cultures in primary care for uncomplicated respiratory infections 1

Treatment Implications

Most viral respiratory infections require only symptomatic treatment (analgesics, intranasal steroids, saline irrigation), while specific antiviral therapy exists only for influenza, HSV, VZV, and HIV. 4, 1

• Oseltamivir should be initiated within 48 hours of symptom onset for confirmed or suspected influenza in high-risk patients 5
• No established antiviral therapy exists for RSV, parainfluenza, adenovirus, metapneumovirus, or most other respiratory viruses in immunocompetent adults 4